This invention relates to a stepped piston for use in a stepped piston internal combustion engine of the kind, hereinafter referred to as the kind specified, having one or more cylinders, the or each cylinder containing a stepped piston having a pumping part of larger diameter, and a working part of smaller diameter, the working part of the piston being slidable in, and associated with, a working part of the cylinder in which combustion of a fuel/air mixture takes place, and the pumping part of the piston being slidable in, and associated with, a pumping part of the cylinder.
Such engines do not rely upon crank case compression for charging the working part of the or each cylinder with the air/fuel mixture, but rather the mixture is delivered from the pumping part of the or each cylinder.
The lubrication of the usual crankshaft and connecting rod bearings, of such engines, and the larger diameter part of the piston and associated cylinder wall is at present provided in a highly rated, high speed, engine, by a lubricant circulatory system incorporating an oil pump and oil feed openings in the surfaces of the bearings to permit passage of lubricant to the bearings. The oil is contained in a sump built into a lower part of the engine or in a separate oil tank connected to the oil pump by feed and return pipes.
Alternatively, lubrication of the bearings in a lower cost, lower speed, engine is provided by oil contained in a sump immediately beneath the crankshaft, the connecting rod being provided with an extension or "dipper" which is partially immersed in the oil in the sump each time the crankshaft is at or near bottom dead centre position thereby causing oil to be splashed onto the bearing surfaces of the engine.
Thus the lubrication of the abovementioned parts is achieved without the complication of the more expensive systems.
In a stepped piston engine, the larger pumping part of the piston must be provided with efficient oil control piston ring(s) in order to avoid excessive oil consumption. The quantity of oil which escapes past the control ring or rings towards the smaller diameter part of the piston and the cylinder wall, is inadequate to provide lubrication between the piston surfaces, the piston rings and the associated cylinder wall of the smaller diameter part of the piston/cylinder and accordingly some further lubrication is required.
The lubrication requirements of the smaller diameter part of the cylinder and piston are similar to those of crankcase compression two stroke engines except for the considerable reduction in piston surface loading in the smaller diameter part of the stepped piston compared with the piston of the crank case compression engine. In the latter engines it is common practice to employ oil mist lubrication provided by either oil mixed into the fuel, or a metered oil supply provided by a precision, variable supply, oil pump driven from the crankshaft, the oil delivery being controlled in relation to the throttle opening of the carbureter.
When these methods of lubrication are used for two stroke engines, there is excessively high oil consumption, particularly compared with four stroke engines. In addition, the commonly seen exhaust smoke of these two stroke engines is unacceptable where exhaust pollution regulations are in force, and is undesirable in any application.
The smaller diameter part of the piston and cylinder of stepped piston engines may be lubricated by the above mentioned oil mist lubrication means, providing a functionally satisfactory engine, but with the same excessive oil consumption problem. This is as undesirable from the market viewpoint as it is when used on crank case compression engines.
Alternative means of lubrication have been proposed in crankcase compression engines using a precision oil metering pump to feed oil separately to the bearings and, in some engines, directly to the cylinder walls. This means can be applied for stepped piston engines by providing a metering pump to supply small quantities of oil direct to the cylinder wall of the smaller diameter part of the cylinder. This is functionally satisfactory but a cost penalty is incurred, since the metering pump and its drive gearing are additional to the normal lubrication system used for lubricating the bearings and large diameter pumping parts of the cylinder and piston.
An example of the use of such a system is provided by rotary piston engines which have been used in a large number of cars. Such engines commonly have a trochoidal piston in an epitrochoidal cylinder. These engines employ a circulation system fed by one oil pump to lubricate the main parts of the engine and a second oil pump of precision metering type, with throttle control variable delivery, to lubricate apex seals of the trochoidal piston and allied components exposed to the combustion process in the engine. The latter example is closely comparable with the lubrication requirements of stepped piston engines and provides the optimum solution to the problem of lubricating this type of engine prior to the means of piston and cylinder lubrication provided by the present invention.